6.6:
Mass Spectrum: Interpretation
Given a known molecular ion mass, the unknown compound can be identified.
The empirical formula is determined from the molecular ion mass and the isotope peak intensities.
This spectrum has a molecular ion peak at m/z 16.
The nitrogen rule states that a molecular ion with even mass contains even or zero nitrogen atoms. Since the molecular ion peak is at 16, it contains no nitrogen.
The number of carbon atoms is deduced by comparing the M+1 intensity to the expected M+1 contribution per carbon atom.
The isotope peak intensities and patterns are also compared to the natural abundances of the isotopes of other likely atoms.
Since this compound contains one carbon and no nitrogen, the empirical formula is CH4.
However, larger compounds need further examination for other atoms, the degree of unsaturation, and fragmentation patterns.
The degree-of-unsaturation formula indicates the presence of multiple bonds or rings, and signature fragments at characteristic m/z values like 15 suggest certain structural features.
6.6:
Mass Spectrum: Interpretation
An unknown compound can be established by identifying the molecular ion peak in the mass spectrum. The molecular ion peak is often weak or absent due to the predominance of fragmentation in high-energy electron beams. In such cases, a low-energy electron beam can be used to scan the spectrum to enhance the intensity of the molecular ion peak. Additionally, chemical ionization, field ionization, and desorption ionization spectra are used to obtain a relatively intense molecular ion peak.
To determine the empirical formula from the molecular ion mass, a high-resolution mass spectrometer like a double-focusing mass spectrometer, a time-of-flight mass spectrometer, an orbitrap mass spectrometer, or a Fourier transform ion cyclotron resonance mass spectrometer is required.
Comparing the intensities of isotope peaks (M+1 and M+2), which arise from the natural abundances of higher-mass isotopes, with the molecular ion peak (M+) gives a fair idea of the empirical formula of an unknown compound.
The number of carbon atoms present in the unknown compound can be deduced by comparing the M+1 to M+ peak intensity ratio to the expected M+1 contribution per carbon atom.
The nitrogen rule, stating that a molecule having an even molecular weight contains zero or even number of nitrogen atoms, predicts the number of nitrogen atoms present in the unknown compound.
Further, the degree of unsaturation predicted from the formula
indicates the presence of multiple bonds or rings in the unknown compound. Lastly, the study of fragmentation patterns for compounds based on a series of guidelines helps identify signature fragments at characteristic m/z values, suggesting the presence of certain structural features.